Check the number of signal and number of sources positions

[astanziola]

The user should receive an error when tries to do something like

sources = Sources(
  positions=((32,),(32,),(32,)),
  signals=jnp.stack([s1, s1]),
  dt=time_axis.dt,
  domain=domain,
)

since the number of signals is not the same as the number of sources positions